Lead Free Rework
Lead Free Rework
Successful rework and PCB assembly methods have been developed with lead-free solders for all types of components. During the transition period to lead-free manufacturing rework practices have to be developed that deal with the issues related to handling, reworking, tracking and inspecting lead free PCB assemblies.
Lead-free solder rework is different than Sn63 solders as the lead-free solder alloys typically do not wet or wick as easily.
on the copper, potentially causing failures by cracking of the copper traces or plated vias. The propensity of the rework operation to cause this cracking is a function of variables such as the PCB layer count and thickness, the laminate material, the rework temperature profile, the copper distribution, the via geometry as well as others.Alternative higher glass transition temperature, lower coefficient of thermal expansion laminate materials may need to be specified for lead-free soldering. Higher rework processing temperatures (approximately 30-40°C higher) may impact other components that are close to their melting or softening points such as connectors, batteries and labels.
The issue of "component mixing" warrants special concern, especially during the transition period. Preliminary studies on the impact of lead in lead-free solder on long-term reliability indicate that the impact varies with the amount of lead in the solder joint, and the impact may be the greatest when the amount of lead is within some intermediate range because of the formation of segregated phases (e.g., coarse lead grains) in the last-to-solidify interdendritic tin grain boundaries, where cracks may initiate and propagate under cyclic loading. For example, it has been shown that 2 to 5 percent lead can be detrimental to the fatigue life of lead-free solder, but it probably is no worse than the Sn63 solder. For example, if a PCB fabricated with Sn63 is to be repaired with lead-free solder, from the solder point of view, the reliability of mixed lead-free solder and Sn/Pb solder will likely not be inferior to the Sn63 solder. However, the temperature impact on the components would be a concern. On the other hand, repairing a lead-free soldered board with Sn63 solder would create solder joints that are not as reliable as the lead-free solder joints on the rest of the board.
In terms of logistics, during the transition period it is key that solder irons and materials (wirecore solder, flux gels, etc.) for lead-free soldering be clearly labeled. Reviewing the JEDEC lead free PCB assembly and component identification guidelines will be required in order to properly mark and identify the materials. Operators must be trained in lead-free rework processes and most importantly inspection. A good review of the IPCA610-D revision will be in required as part of this training.
BEST has several tools that will get you “up to speed” in terms of lead-free rework including:
- Lead-Free Evaluation Kits which include a practice board, practice components as well as a spool of lead-free solder to practice your hand-soldering rework practices
- Lead-Free oriented training courses including BEST’s well-known SMT and advanced SMT courses which have their lead-free versions. Look for BEST lead-free interactive CD and web-based training modules coming soon!
- Lead-Free rework including BGA reballing, removal and replacement services